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<h1>Events and Listeners</h1>Events in GWT use the "listener interface" model similar to other user
 interface frameworks. A listener interface defines one or more methods
 that the widget calls to announce an event. A class wishing to receive
 events of a particular type implements the associated listener interface
 and then passes a reference to itself to the widget to "subscribe" to a
 set of events.
 
 <p>
 The <a xmlns="" href="com.google.gwt.user.client.ui.Button.html">Button</a> class, for example,
 publishes click events. The associated listener interface is
 <a href="com.google.gwt.user.client.ui.ClickListener.html">ClickListener</a>.
 </p>
<pre class="code">public void anonClickListenerExample() {
  Button b = new Button("Click Me");
  b.addClickListener(new ClickListener() {
    public void onClick(Widget sender) {
      // handle the click event
    }
  });
}
</pre>

 
 Using anonymous inner classes as in the above example can be inefficient
 for a large number of widgets, since it could result in the creation of
 many listener objects. Widgets supply their <code>this</code> pointer
 as the <code>sender</code> parameter when they invoke a listener
 method, allowing a single listener to distinguish between multiple event
 publishers. This makes better use of memory but requires slightly more
 code, as shown in the following example:
 
 <pre class="code">public class ListenerExample extends Composite implements ClickListener {
  private FlowPanel fp = new FlowPanel();
  private Button b1 = new Button("Button 1");
  private Button b2 = new Button("Button 2");

  public ListenerExample() {
    initWidget(fp);
    fp.add(b1);
    fp.add(b2);
    b1.addClickListener(this);
    b2.addClickListener(this);
  }

  public void onClick(Widget sender) {
    if (sender == b1) {
      // handle b1 being clicked
    } else if (sender == b2) {
      // handle b2 being clicked
    }
  }
}
</pre>

 
 Some event interfaces specify more than one event. If you are only
 interested in a subset of these events, subclass one of the event
 "adapters". Adapters are simply empty concrete implementations of a
 particular event interface, from which you can derive a listener class
 without having to implement every method.
 
 <pre class="code">public void adapterExample() {
  TextBox t = new TextBox();
  t.addKeyboardListener(new KeyboardListenerAdapter() {
    public void onKeyPress(Widget sender, char keyCode, int modifiers) {
      // handle only this one event
    }
  });
}
</pre>
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